The present invention relates to a subsea flexible pipe for transporting oil and gas in deep waters.
Flexible pipes for transporting oil and gas are already well known, and they generally comprise, from the inside to the outside of the pipe, a metal carcass, an inner impervious polymer sheath, a pressure arch, laps of traction armor, and an outer polymer sheath for protecting the overall pipe and in particular for preventing seawater from entering into its thickness. The metal carcass and the pressure arch consist of longitudinal elements wound in a short pitch, and they give the pipe its resistance to radial forces, while the laps of traction armor consist of metal wires wound in long pitches to absorb the axial forces. The type, number, dimensions and organization of the layers constituting the flexible pipes are essentially related to their conditions of use and installation. In the present application, the concept of short pitch winding designates any helical winding at a helix angle close to 90°, typically between 75° and 90°. The concept of long pitch winding covers helix angles lower than 55°, typically between 25° to 55° for the armor laps.
These flexible pipes are suitable for transporting oil and gas, in particular on the seabed, and at great depths. More precisely, they are referred to as unbonded and are so described in the standards published by the American Petroleum Institute (API), API 17J and API RP 17B.
When the flexible pipe, regardless of its type, is subjected to an external pressure that is higher than the internal pressure, an axial compression may be generated, which is known to a person skilled in the art as the reverse end cap effect. The reverse end cap effect tends to compress the flexible pipe axially, to shorten its length and to increase its diameter, thereby tending to cause a swelling of the traction armor laps. In the case in which the outer sheath of the pipe is impervious, the hydrostatic pressure prevailing outside the pipe effectively opposes the swelling of the traction armor. However, if the outer sheath is no longer impervious, for example due to an accidental tear, the hydrostatic pressure no longer opposes the swelling of the traction armor laps. In consequence, in the absence of additional means for limiting this swelling, the wires of the traction armor laps may buckle in a radial mode, thereby possibly causing an irreversible local deformation of said armor laps having a “birdcage” shape, and thus causing the failure of the pipe.
One known solution for reducing this “birdcage” radial buckling risk consists of a short pitch winding, around the traction armor laps, of bands reinforced with aramide fibers, and more precisely, fibers sold under the Kevlar® trademark by du Pont de Nemours. Such bands have high tensile strength along their longitudinal axis, thereby serving to limit the swelling of the traction armor laps. They also have great bending flexibility, thereby facilitating the handling and winding operations around the armor laps. Finally, at equivalent mechanical characteristics, they are much lighter than metal bands, thereby serving to reduce the weight of the flexible pipe. Reference can be made in particular to document FR 2 837 899 in which such a pipe is disclosed.
These reinforcing bands are in the form of bundles of fiber strands or filamentary strands of Kevlar® oriented parallel to the longitudinal axis of the band. These longitudinal fiber strands may be joined to one another in the form of a relatively flat bundle having a substantially rectangular cross section like that of a ribbon or a tape. It is also possible to use a reinforcing band consisting of a substantially rectangular central section and two longitudinal edges that are thinner than the central section as described in document EP1419338. The means for joining and retaining these fiber strands or filamentary strands generally comprise crosswise elements that are shaped so as to surround and grip all of said strands in order to form a relatively flat bundle. In common configurations, these crosswise elements are treated as weft yarns, the filamentary strands forming the warp, and the band can then be considered as a woven material. Various embodiments of these reinforcing bands are described in documents WO97/12753 and WO9713091.
However, it has been found, despite this, that in extreme service conditions these reinforcing bands could be damaged. These extreme conditions are mainly encountered when the flexible pipe is, on the one hand, submerged to great depth, typically over 2000 m, and on the other hand, simultaneously subjected to dynamic bending loads, causing fatigue of the reinforcing bands. These conditions may be satisfied in the lower portion of the flexible risers arranged in a catenary, and connecting the seabed with a floating support at the surface. Due to the movements of the floating support, the lower part of the catenary may be subjected to wide variations in curvature. Moreover, this dynamic zone is located close to the touch down point, that is, potentially at great depth.